The present invention relates to structure for wetting a diaphragm of an electrochemical cell employing a solid polymer electrolyte (hereinafter referred to as "PE") for attaining a high performance, and a process of preparing same.
Especially a fuel cell among electrochemical cells employing PE has been developed as an anti-pollution electric power generator having high energy efficiency in view of the problems of exhaustion of energy resources and global environmental pollution.
A fuel cell employing PE is expected to be a lightweight and high power generator which may be useful as a small-size electric source for an electric automobile or the like (disclosed in reference 1; D. Watkins et al., Proceeding of the 33rd International Power Sources Symposium, PP. 782 to 791, 13 to 16, Jun. (1988), and in reference 2; E. A. Ticianelli et al., J. Electrochemical Society, 135, 2209, (1988)).
Other electrochemical cells using PE are expected to be employed in electro-synthesis starting from water, methanol, such a hydrocarbon as ethylene and the like in the future.
The performance of these cells is largely influenced by the ionic conductivity of the PE employed.
When a sufficient amount of water is not contained in a PE, its conductivity considerably decreases with the increase of its resistance to lower the cell performances.
In order to overcome the said drawback, a reaction gas (a hydrogen gas as fuel and oxygen or air as oxidant) containing water vapor may be introduced into a cell.
Since, in this improved cell, the partial pressure of the reaction gas decreases due to dilution with the water vapor by the partial pressure of the water vapor, and the diffusion of the reaction gas in the reaction layer of an electrode is also reduced, the characteristics of the cell may be deteriorated.
Since the water pressure which changes with a changing load cannot be sufficiently controlled, the PE is dried or the catalyst layer is inversely overwetted resulting in the deterioration of the cell performance.
A larger amount of H.sub.2 O is exhausted in an anode side because several molecules of H.sub.2 O are accompanied with an H.sup.= ion permeating through the PE from the anode to the cathode of which the amount is proportional to the current. However, it is difficult to overcome this disadvantage by the said water vapor mixing process (disclosed in reference 3; T. Springr et al., Extended Abstract No. 118, J. Electrochemical Society Meeting, vol. 190-2, Oct. (1990)).
Accordingly, these problems shall be absolutely overcome for employing the fuel cell as a moving electric source of which a load largely changes, and the same problems exist in an electrolytic cell using a PE.
While water is directly supplied to the rear of an anode and electrolyzed in conventional water electrolysis, an extra operation of separating the oxygen gas evolved and the unreacted water outside of the cell is required.
In order to overcome these drawbacks, the present inventor has proposed an improved structure of wetting a diaphragm in my prior application (U.S. Ser. No. 07/835,222). The application has provided the structure of PE in which a plurality of yarns made of, for example, anti-corrosive polytetrafluoroethylene for supplying water through the yarns are formed. Although, however, this structure is effective for elevating the cell performances, the supply rate is sometimes insufficient to achieve the desired cell performances because the water or water vapor proceeds while passing through the yarns.